Aims: We investigated cancer incidence in a large cohort of underground uranium miners largely employed when exposure levels in the mines were relatively low. In addition to providing data on low-level exposures, by studying incident cancers rather than mortality and collecting data on smoking and dust exposure, our study should provide a more complete picture of the risk associated with radon exposure. Procedures and techniques: We studied male uranium miners from Pribram, Czech Republic, who had worked at least one year underground prior to January 1, 1976, and were alive and living in the Czech Republic on January 1, 1977. We computerized industry records from 1945-1992, when mining ceased. Available data included a unique identification number that is assigned at birth and start and stop dates of employment in the mines. Many sources were used to establish vital status and residence on January 1, 1977 for all but 2.7% of eligible miners, including a register of inhabitants, uranium industry records, vital records, and the Czech and Slovak cancer registries. Of 18,985 miners, 16,434 were found to be eligible and were followed for cancer incidence and mortality through December 31, 1992 by linking the computerized miner database with the Czech cancer registry (for which records were computerized since 1976), the Pribram region cancer registry (paper records from 1976), the Slovakian cancer registry and a combined Czech and Slovakian registry, death certificates, and uranium industry records. More detailed exposure data, including daily records of work location and pertinent radon levels, smoking, dusts, and gamma radiation were obtained for a 2,000-person stratified random sample (stratified on age and decade of mining) of the cohort and all cancer cases for use in case-cohort analyses. Cancer follow-up was extended through 1998 to increase the number of miners with cancers of sites other than the lung. Detailed exposure data have been obtained for these additional miners and records are now being computerized and verified. Accomplishments: Cancer incidence and mortality rates though 1992 have been estimated and compared with rates for the general Czech Republic and with rates for the Pribram region. Cancer risk among miners also has been evaluated by comparing cancer rates among miners with higher exposures to the rates among miners with the least amount of radon exposure. Lung cancer was diagnosed in 752 miners. Fifty-six percent of miners began mining after 1960 when industrial hygiene measures substantially reduced radon exposure. Cumulative exposure was less than 50 WLM for 72% of the cohort. Compared to incidence rates for the Czech Republic, miners had a more than two-fold risk of developing lung cancer. Although risk was not increased for miners with fewer than 5 WLM, the risk was significantly increased at all higher exposure categories and increased steadily with dose. Age- and calendar year-adjusted relative risks comparing exposed miners to those with fewer than 5 WLM showed a similar trend. Preliminary results from the case-cohort analysis confirm that the radon-associated risks are not explained by smoking or by occupational exposure to dusts in the mines. Cancer of sites other than the lung was diagnosed in 1100 miners. The overall standardized incidence for all non-lung cancers was slightly elevated. There were no consistent trends with duration of underground employment or cumulative working level months of exposure. Risk was increased, however, for specific cancers. For example, there was a 50% increase in leukemia, that was statistically significant, and risk was increased at all exposure levels above 50 WLM. There was no overall increase in risk for kidney cancer, except at doses greater than 200 WLM. Significantly increased standardized incidence ratios were also found for cancers of the larynx, pancreas, liver, stomach, and colon and rectum, but the absence of consistent dose-response trends points to factors other than radiation. Our data provide much needed information on exposure levels that are more reflective of exposures found in residences. We are the first to study cancer incidence rather than mortality and will have the opportunity to directly compare the results when incidence or mortality data are used. Since we have data on smoking and other exposures in the mines, we will also be able to explore the influence of potential confounders - something that has not been possible for most cohorts.